The present invention relates to optical reflectometry, and more particularly to a method and apparatus for measuring the surface geometry of an image sensor mounted in its package.
In many applications for digital imagers there is a need to obtain a high degree of flatness. There is a need for an improved method of assessing the flatness of such digital imagers. In digital Single Lens Reflex (SLR) cameras there is a need to maintain a high degree of flatness of the imager plane so that the camera can be accurately focused. It is important that the flatness of the packaged imager be accurately measured so that the focus capability of the imager can be assessed. In the case of digital imagers for digital radiography applications, scintillating fiber optic faceplates are usually placed in contact with and attached to the surface of the digital imager. There is a requirement that the maximum gap between the fiber optic faceplate and the digital imager be small. It is important to know the surface profile of the imager surface so that the fiber optic faceplates can be ground to match the surface of the imager to tight tolerances. In particular it is desired to calculate the best-fit spherical surface equivalent for the imager surface and grind the fiber optic faceplates to the same sphere and maintain this tight tolerance. A better solution is to assemble the digital imager so that it is flat enough to directly mate to a flat surface of a ground fiber optic faceplate. In order to be able to get to the required levels of flatness, an assessment of the flatness of the package, imager chips and imager gluing process also needs to be understood.
Prior art methods and apparatus for measuring flatness of substrates are known in the art as follows: U.S. Pat. No. 6,323,952, issued Nov. 27, 2001 to Yomoto et al., describes a flatness measurement apparatus including a TV camera coupled to a Fizeau interferometer. The apparatus operates using the method of tracking fringes and a fringe contrast judging criteria is utilized that requires a high degree of visibility of the fringes. There is also the problem of assessing height steps in the substrate. Also, this method is limited to measuring a single surface at a time.
U.S. Pat. No. 6,321,594 B1 issued Nov. 27, 2001 to Brown et al., describes a laser triangulation method for assessing bulging or bowing of thin films. This approach is also limited to measuring a single surface at a time.
U.S. Pat. No. 5,402,234 issued Mar. 28, 1995 to Deck, describes a coherence scanning interferometry-based microscope including a CCD camera and a constant velocity z-axis stage coupled to the sample for scanning the depth of the sample. The sample is in one leg of the interferometer and the relative intensity of the interference peak is analyzed as a function of height of the transport stage using an interference discriminator. Only one surface at a time can be analyzed by this method.
Methods for simultaneously measuring the thickness and group index of refraction of films using low coherence light interferometry in an autocorrelation configuration are also known in the prior art. For the purposes of this discussion, an interferometer operating in an autocorrelation configuration is defined to be an interferometer having a variable differential time delay. One embodiment of an optical autocorrelator is described, for example, in chapter 5 of Statistical Optics, by Joseph W. Goodman (John Wiley and Sons, 1985, pp. 157-170). Those skilled in the art are aware of the principles of operation of an optical autocorrelator, but certain principles will be clarified here because of their relevance to the present invention. In an interferometer operating in an autocorrelator configuration wherein light is split into two different paths and then recombined and directed to a photodiode, the detected light intensity is measured as a function of a parameter. This parameter can be the differential optical path length xcex94L of the interferometer or it can be the differential time delay xcex94t of the interferometer. These parameters are related by xcex94L=ncxcex94t, where c is the speed of light in vacuum and n is the group index of refraction of the medium (usually air) of the differential optical path. The detected light intensity expressed as a function of the differential time delay is called the coherence function of the input light. Hence, a receiver which determines the time delay between light reflected from different surfaces of a film performs the same function as a receiver which determines the path delay between light reflected from different surfaces of a film. Determining the spacing between peaks in the coherence function of the reflected light is yet another way to describe the same function. For the purposes of the present discussion, the term differential time delay shall include differential path delay.
A Michelson interferometer is an example of such an interferometer operating in an autocorrelation configuration. An example of an apparatus for measuring film thickness utilizing a Michelson interferometer is taught in U.S. Pat. No. 3,319,515 issued May 16, 1967 to Flournoy. In this system, the film is illuminated with a collimated light beam at an angle with respect to the surface of the film. The front and back surfaces of the film generate reflected light signals. The distance between the two reflecting surfaces is then determined by examining the peaks in the autocorrelation spectrum generated in a Michelson interferometer that receives the reflected light as its input.
U.S. Pat. No. 5,633,712 issued May 27, 1997 to Venkatesh et al., discloses a method and apparatus for simultaneously determining the thickness and group index of refraction of a film using low-coherence reflectometry in an autocorrelation configuration. The apparatus includes a low coherence light source that generates a probe light signal. The film is positioned between first and second reference reflectors, the first reference reflector being partially reflecting. The probe light signal is applied to the film after passing through the first reference reflector. The portion of the probe light signal leaving the film is reflected back toward the first reference reflector by the second reference reflector. The light exiting through the first reference reflector is collected to form the input to a receiver that determines the time delay between light reflected from the top and bottom surfaces of the film as well as the change in optical path length between said first and second reflectors resulting from the introduction of said film between said first and second reflectors.
Prior art methods for measuring the surface profile of a sample include the use of contact profilometry, which employs a probe to physically contact the surface of the sample and generate a surface profile. Non-contact methods for surface profile measurement include optical phase shifting interferometers as described in U.S. Pat. No. 4,955,719 issued Sep. 11, 1990 to Hayes, and vertical scanning interference microscopy as described in U.S. Pat. No. 5,446,547 issued Aug. 29, 1995 to Guenther et al. These traditional non-contact methods require turning the sample over and engaging in edge and corner alignment in an attempt to measure the top and bottom surface profiles of corresponding locations.
There is a need therefore to provide an improved apparatus and method to measure surface profiles and orientation of surfaces, such as the surfaces of a digital imager in its package, the effects of wafer processing conditions, bonding and packaging on the resultant flatness of an image sensor light receiving surface.
The need is met according to the present invention by providing apparatus for measuring the surface profile of a sample that includes a fixture for locating a surface of a transparent optical flat relative to a surface of a sample; a low-coherence light interferometer having an optical probe coupled to an XY scanning frame for scanning the surface of the sample through the transparent optical flat to produce interferometric data signals representing the distances between the optical flat surface and the surface of the sample; and a computer system responsive to the interferometric data signals for generating a surface profile of the sample using a best fit to a plane.
The measurement apparatus and method of the present invention has the advantage that it can be used during imager curing for assessing the cure cycle of adhesives used to bond digital imagers to packages and to observe wafer bow arising from manufacturing process steps and also to evaluate package flatness. Measurements of a 2xe2x80x3 by 2xe2x80x3 surface can be made in minutes.